Dependent Relationship between Quantitative Lattice Contraction and

Subscriber access provided by LAURENTIAN UNIV

Article

Dependent Relationship between Quantitative Lattice Contraction and Enhanced Oxygen Reduction Activity over Pt-Cu Alloy Catalysts Yige Zhao, Yijun Wu, Jingjun Liu, and Feng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08437 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Dependent Lattice

Relationship

Contraction

and

between

Quantitative

Enhanced

Oxygen

Reduction Activity over Pt-Cu Alloy Catalysts Yige Zhao, Yijun Wu, Jingjun Liu*, Feng Wang* State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029 (China) ABSTRACT: Lattice contraction has been regarded as an important factor influencing oxygen reduction reaction (ORR) activity of Pt-based alloys. However, the relationship between quantitative lattice contraction and the ORR activity has rarely been reported. Herein, using PtCu alloy nanoparticles (NPs) with similar particle sizes but different compositions as examples, we investigated the relationship between quantitative lattice contraction and the ORR activity by defining the shrinking percentage of Pt-Pt bond distance as lattice contraction percentage. The results show that the ORR activities of Pt-Cu alloy NPs exhibit a well-defined volcano-type dependent relationship toward the lattice contraction percentage. The dependent correlation can be explained by the Sabatier principle. This study not only proposes a valid descriptor to bridge the activity and atomic composition but also provides a reference for understanding the composition-structure-activity relationship of Pt-based alloys. KEYWORDS: Pt-based alloy, dependent relationship, lattice contraction percentage, oxygen reduction reaction, X-ray absorption spectroscopy

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.

Page 2 of 27

INTRODUCTION Polymer electrolyte membrane fuel cells have been regarded as promising alternatives to fossil

fuels by virtue of the efficient energy conversion and low emissions.1-2 However, their widespread application is still hindered by the sluggish oxygen reduction reaction (ORR).

3-4

Platinum has been considered as the most common and efficient catalyst to accelerate the ORR at the present stage.5 Nevertheless, its high cost and low stability make investigators to seek for more inexpensive and stable catalysts. So far, extensive efforts have been dedicated to reducing Pt usage by alloying Pt with another transition metal such as Cu, Co, Ni, Fe and so on.6-7 This strategy can not only decrease the cost by reducing Pt content but also improve the ORR activity and stability relative to pure Pt.8-9 The enhanced ORR performance induced by the alloying of transition metals can be accounted for two effects. One is the geometric or strain effect referring to the change of the Pt-Pt bond distance,10-11 and the other is the electronic or the ligand effect that refers to the change of electronic structure induced by hetero-metallic bonding interactions between Pt and M (M referring to another transition metal).12-13 These above two effects can be interpreted by the dband theory of noble metals developed by Nørskov et al.14-15 It demonstrates that the d-band center of Pt or Pt-O binding energy in the ORR process plays a very significant role in improving the ORR activity. The theory is beneficial for our understanding for the nature of enhanced ORR kinetics. However, the d-band center of Pt or Pt-O binding energy can only serve as an effective activity descriptor with well-characterized surfaces.16 Considering the fact that the surface composition and structure of Pt-M alloy prepared by the conventional method like polyol approach are often complicated, a more practical activity descriptor than the commonly used


2

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


alloy composition is necessary to develop an in-depth comprehension of the origin of ORR activity at the atomic level. It is well-known that the incorporation of M atom into Pt lattice for Pt-M alloys brings about the lattice contraction (the decrease of Pt-Pt bond distance) when M atoms have smaller atomic radius than Pt atoms.

13, 17-19

The opinion that the lattice contraction contributes to the ORR

kinetics has been proved by many previous reports. For example, Strasser et al. showed that the activity of Pt-Cu nanoparticles increased with Cu content due to the increasing lattice strain.

20

Ying et al. suggested that the extraordinary ORR activity of the AuCu@Pt nanoparticles was ascribed to the compressive strain effect resulted from the AuCu alloy core with slightly smaller lattice parameter than that of Pt shell.

21

Apart from that, Lu et al. found that the compressive

strain could weaken the Pt-O binding and increase the ORR activity on Pt-Cu core/shell nanoparticles.22 In this case, lattice contraction can serve as an appropriate activity descriptor. On the one hand, it is easy to be controlled just by regulating the alloy composition; On the other hand, it can go deep into the atomic level to bridge the activity and atomic composition, which helps us to acquire more knowledge on the structural factors influencing the ORR kinetics in order to obtain the most optimal catalyst structure with the highest activity. It is worth mentioning that the dependent relationship between quantitative lattice contraction and enhanced oxygen reduction activity has rarely been reported. To follow an applicable principle for improving Pt-based catalysts, correlating the electrochemical activity to a quantitative indicator is necessary. Based on the above analysis, connecting quantitative lattice contraction with ORR activity is worth studying. In this work, taking Pt-Cu alloy nanoparticles (NPs) with similar particle sizes but different compositions as examples, we researched the dependent relationship between quantitative lattice


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

contraction and electro-catalytic activity by quantifying the shrinking percentage of Pt-Pt bond distance, which was determined by the Fourier transformed (FT) k3χ (k)-R space curves derived from the X-ray absorption spectroscopy (XAS) of Pt-Cu alloys. The electrochemical tests suggest that the ORR activities of Pt-Cu alloy NPs exhibit a more well-defined relationship toward the lattice contraction percentage than the alloy composition. X-ray photoelectron spectroscopy (XPS) and XAS measurements demonstrate that Pt 5d electrons increase along with the increase of lattice contraction percentage, resulting in the downshift of d-band center, further weakening the adsorption strength of oxygen-containing species. In the beginning, as the lattice contraction percentage increases, the Pt-Pt bond distance declines, the active oxygen tends to be adsorbed on catalyst surface in a “bridge side-on two sides” style, which is helpful for the breakage of O-O bond to boost the ORR activity. Moreover, the adsorption of the oxygenated intermediates becomes weaker on account of the weaker adsorption strength and enhanced repulsive force of oxygenated intermediates. Thus, the ORR activity improves since the weaker binding of oxygenated intermediates releases more active sites. However, when beyond the critical lattice contraction percentage, the oxygen adsorption becomes weaker, limiting the ORR rate. Thus, the correlation between lattice contraction percentage and ORR activity represents a volcano type. This work proposes an effective descriptor to bridge the ORR activity and alloy composition, which is beneficial for understanding deeply the composition-structure-activity relationship of Pt-based alloys. 2. EXPERIMENTAL SECTION 2.1. Preparation of Pt-Cu/C Catalyst: a series of Pt-Cu/C samples were synthesized through the classical polyol method. Firstly, carbon black (100 mg, Vulcan XC-72, Cabot) was dispersed in ethylene glycol (50 ml), followed by the addition of H2PtCl6 (0.01 M) and CuCl2 (0.1 M)


4

Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solutions to the mixed suspension. Then, the suspension was subjected to ultrasonic vibration for 30 min. In the meanwhile, the solution pH value was adjusted to 10 by moderate NaOH (1 M) aqueous solution. Afterwards, the resultant solution was heated at 130 ℃ for 3 h with constant stirring to achieve the co-reduction of the two metals. Finally, the mixture was filtered, washed by deionized water and dried at room temperature to obtain the goal sample 2.2. Physical Characterizations: The morphology was explored by using scanning electron microscopy (SEM) on a JEOL JSM-6701F and transmission electron microscopy (TEM) on a JEOL JSM-2100 microscope. The structure analysis was performed by X-ray diffraction (XRD) on a Philips Xpert X-ray diffractometer with Cu Kα (λ=1.5406 Å). Inductively coupled plasma mass spectrometry (ICP−MS) was used to determine the catalyst compositions. Moreover, X-ray photoelectron spectroscopy (XPS) gained from a Thermo ESCALAB 250 spectrometer with a monochromator (Al Kα source) and X-ray absorption spectroscopy (XAS) measurements were conducted to obtain the information about electronic structures. The XAS at the Pt L3-edge was measured on the 1W1B-XAFS beamline of Beijing Synchrotron Radiation Facility (BSRF), China. The electron beam energy of the storage ring was 2.5 GeV with a stored current of 200 mA. The incident beam was collimated by slits and monochromatized with Si (111) double crystal. The energy was calibrated by a standard Pt foil measured in the transmission mode, while the catalyst samples were measured in the fluorescence mode. The obtained XAS spectra were pre-edge background subtracted and post-edge normalized using ATHENA software. 2.3. Electrochemical Measurements: the electro-catalytic properties were studied by a typical three-electrode cell on Pine Instruments. Cyclic voltammetry (CV) was recorded in N2saturated HClO4 (0.1 M) at 50 mV s-1. The specific electrochemical surface areas (ECSAs) of all the catalysts were calculated by evaluating the areas in the hydrogen adsorption/desorption


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 27

region assuming a factor of 210 µC cm−2 for the adsorption of a hydrogen monolayer. The ORR activity and stability were researched by rotating ring-disk electrode (RRDE) in O2-purged KOH (0.1 M). The commercial Pt/C catalyst (20 %, Johnson Matthey) was measured under the same condition as a comparison. With regard to the fabrication of catalyst ink, catalyst powder (2 mg) with alcohol (1 mL) and Nafion (50 µL, 5 wt.%) were mixed ultrasonically. Afterwards, 10 µL of the resultant ink was pipetted onto the glassy carbon electrode to form the working electrode. The counter electrode was Pt foil and reference electrode was saturated calomel electrode (SCE). The electron transfer number (n) and hydrogen peroxide production yield (H2O2%) were evaluated according to the listed formulas：23

=

4 + ( / )

% = 100 ×

4− 2

where Id, Ir and N refer to the disk current, ring current and current collection efficiency (0.37), respectively. 3. RESULTS AND DISCUSSIONS 3.1 Morphologies and structures for as-prepared Pt-Cu/C catalysts


6

Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. TEM images of Pt-Cu/C alloy NPs with different compositions (Inset: HRTEM images and particle size distribution histograms): (A) Pt81Cu19; (B) Pt72Cu28; (C) Pt67Cu33; (D) Pt55Cu45; (E) Pt44Cu56 and (F) Pt25Cu75. The carbon-supported PtxCu100-x NPs with different Pt contents were fabricated through regulating Pt and Cu atomic ratios. According to the practical element compositions confirmed by inductively coupled plasma mass spectrometry (ICP−MS), the as-prepared samples are referred to as Pt81Cu19, Pt72Cu28, Pt67Cu33, Pt55Cu45, Pt44Cu56 and Pt25Cu75, respectively. Figure 1 represents typical transmission electron microscopy (TEM) images of these catalysts. We can see that the Pt-Cu NPs with a narrow size range are dispersed evenly on carbon surface, without apparent agglomeration (Figure S1). Besides, the six Pt-Cu/C catalysts own the similar average grain diameters, as proved by the size distribution histograms in the insets and the resultant sizes (Table S1) calculated from the XRD spectra (Figure 2A). However, the particle sizes calculated


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

from TEM and XRD techniques are different, which can be explained by the fact that TEM images display the actual morphology of catalysts but XRD only embodies the crystalline particles.24 Moreover, from the high resolution TEM (HRTEM) images of Figure 1, the alloy NPs are crystalline with inter-planar spacing of less than 0.23nm, which is consistent with the cubic Pt (111) lattice plane, suggesting that Cu atoms can incorporate into Pt lattice and finally form the solid solution alloy. To further confirm the formation of well-alloyed structure, X-ray diffraction (XRD) characterizations were conducted on the Pt-Cu/C catalysts with different compositions, as shown in Figure 2A. Except for the carbon (002) plane located at about 25°, the other five peaks corresponds to the (111), (200), (220), (311) and (222) planes of face-centered cubic (fcc) crystalline Pt. In addition, these peaks are slightly shifted to higher angles relative to pure Pt phase without peaks for pure Cu or its oxides appearing, demonstrating that Cu is integrated into Pt lattice to form a single-phase solid solution alloy with a lattice contraction because of the relatively smaller lattice constant of Cu than Pt. Considering that the alloying degree is an important factor connected with the ORR performance,25 we further investigated the relationship between lattice constants and Cu atomic content for the synthesized alloys. The lattice constants were evaluated from the (111) peak positions in the XRD patterns and the final results were represented in Figure S2. It can be obviously seen that the lattice constant decreases linearly as Cu content increases, indicating the increase of lattice contraction, which is caused by more Pt atoms displacement by Cu atoms. Moreover, the calculated lattice constants are located in the neighborhood of the regression line on basis of Vegard’s law, further revealing the homogeneous solid solution alloy structure of Pt-Cu NPs.26


8

Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (A) XRD patterns of Pt-Cu/C samples; (B) Pt L3-edge XANES spectra of Pt-Cu/C and the commercial Pt/C catalysts; (C) k3-weighted Fourier transform spectra acquired from (B); (D) lattice contraction percentage (δ) reflected from the change of Pt-Pt bond distance; (E) schematic illustration of lattice contraction in Pt-Cu alloys. To obtain the direct evidence of the indicated lattice contraction above, we conducted X-ray absorption spectroscopy (XAS) measurements for these catalysts and the normalized X-ray


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

absorption near edge structure (XANES) for Pt L3-edge are shown in Figure 2B. Figure 2C describes the Fourier transformed k3χ (k)-R space curves derived from the XAS spectra, in which the peak at about 2.61 Å is ascribed to the Pt-Pt peak, which is significant in the Pt/C spectra.27 As observed, with the increase of Cu content in the alloy compositions, the Pt-Pt bond distance decreases, confirming the existence of lattice contraction. The concrete Pt-Pt bond lengths (RPt-Pt) of these catalysts were measured and displayed in Figure S3. To quantify the lattice contraction, we define the contraction percentage of Pt-Pt bond distance as the lattice contraction percentage (δ), and it can be calculated based on the equation (1):

δ (%) =

∣ R − R ∣ × 100 (1) R

where Ralloy is the Pt-Pt bond length in the Pt–Cu/C alloy catalysts, RPt is the Pt-Pt bond length in the Pt/C catalyst. Based on the above formula, the lattice contraction percentages of all the catalysts were calculated and shown in Figure 2D. We can observe that the lattice contraction percentages of Pt81Cu19, Pt72Cu28, Pt67Cu33, Pt55Cu45, Pt44Cu56 and Pt25Cu75 catalysts are 1.15%, 2.68%, 3.83%, 4.60%, 5.36%, 6.90%, respectively. Thus, these alloy catalysts can be marked by δ-1.15, δ-2.68, δ-3.83, δ-4.60, δ-5.36, δ-6.90, respectively. Obviously, the lattice contraction percentage increases as the Cu atomic ratio increases. It suggests that Cu incorporating into Pt lattice causes the lattice contraction, as illustrated by Figure 2E. 3.2 Correlation between lattice contraction percentage and the ORR activity


10

Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (A) ORR polarization curves recorded at 1600 rpm with a scan rate of 10 mV s-1 in O2saturated 0.1 M KOH solution; (B) half-wave potentials of these catalysts; (C) mass activities and specific activities of these catalysts. The correlation between lattice contraction percentage and the ORR activity was systematically studied by a rotating disk electrode (RDE) in O2-saturated 0.1 M KOH solutions. Figure 3A represents the acquired ORR polarization curves on these Pt-Cu alloys with different lattice contraction percentages recorded at 1600 rpm with a sweep rate of 10 mV s-1, which were normalized by the glassy carbon area (0.247cm2). The commercial Pt/C catalyst (JM, 20wt% loading) serves as the reference material. The half-wave potential (E1/2) in the mixed kinetic/diffusion region is often applied to assess the ORR activity.28 As shown in Figure 3B, the half-wave potentials follow the order: δ-3.83 > δ-2.68 > δ-4.60 > δ-1.15 > δ-5.36 > Pt/C > δ-6.90. Interestingly, the ORR activities display a volcano type as a function of lattice contraction


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

percentage. This phenomenon reveals that the electro-catalytic activities heavily rely on lattice contraction percentage. Moreover, the mass activities (MA) of all the samples were calculated through normalizing the kinetic currents by Pt loading amount on the electrode. Simultaneously, the specific activities (SA) were also figured out through normalizing the MA by specific electrochemical surface areas (ECSAs). The ECSA values were calculated by evaluating the areas in the hydrogen adsorption/desorption region of the cyclic voltammetry (CV, Figure S4) assuming a factor of 210 µC cm−2 for the adsorption of a hydrogen monolayer,29 as displayed in Figure S5. As plotted in Figure 3C, a similar volcano type is seen for both MA and SA as a function of lattice contraction percentage. The optimum δ-3.83 catalyst exhibits a mass activity of 4.4 times (0.48 mA µg-1) that of Pt/C (0.11 mA µg-1) and a specific activity of 4.1 times (0.53 mA cm-2) that of Pt/C (0.13 mA cm-2), respectively. Notably, this dependent relationship between the ORR activities and lattice contraction percentage is more well-defined than that between the ORR activities and alloy compositions shown in Figure S6, manifesting that the lattice contraction percentage can serve as a more feasible activity descriptor than the alloy composition. Furthermore, the lattice strain values of these Pt-Cu/C samples were also calculated according to the equation (2): 30

strain (%) =

a − a × 100 (2) a

where a is the lattice constant for Pt/C catalyst, a is the lattice constant for Pt–Cu/C alloy catalysts. The lattice strain results are shown in Figure S7. We can see that the quantitative relationship between the ORR activities and lattice strain (Figure S8) are similar to that between the ORR activities and lattice contraction percentage (Figure 3C). Taken together, moderate lattice contraction plays a significantly positive role in ORR activity improvement, but excessive


12

Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lattice contraction reduces the activity. The detailed reason will be analyzed by revealing the intrinsic property of these catalysts in the following section 3.3.

Figure 4. (A) Rotating ring-disk electrode voltammograms measured in O2-saturated 0.1 M KOH solution at1600 rpm; (B) the transferred electrons number over these catalysts; (C) the hydrogen peroxide yields over these catalysts. To obtain a profound understanding of the ORR pathway, we further carried out rotating ringdisk electrode (RRDE) tests in O2-saturated 0.1 M KOH solutions. As is seen in Figure 4A, the measured disk currents are much larger than the ring currents for all the catalysts, indicating that the ORR catalytic process may undergo a four-electron pathway. The transferred electron number (n) and hydrogen peroxide yield (H2O2%) were determined, as displayed in Figure 4B and C, respectively. Especially, for the best δ-3.83 catalyst, its n value remains ca. 4 in the whole range of scanning potentials. Besides, the H2O2% of the δ-3.83 catalyst stays around 2%, much less than that of Pt/C catalyst (4%~7%), suggesting less intermediate products during ORR.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

Interestingly, in the potential ranging from 0.4 to 0.6 V, the n and H2O2% values of all the tested samples during the ORR process also represent a volcano type as a function of lattice contraction percentage, further demonstrating the importance of lattice contraction in Pt-Cu alloys. Apart from that, the significant role of lattice contraction have been also demonstrated in other Pt-M (M=Fe, Ni, Co) alloys,16, 31-32 indicating that the lattice contraction is related to not only the atomic composition but also the identity of the second element. 3.3 Origin of the dependent relationship between activity and lattice contraction percentage

Figure 5. (A) Pt 4f XPS spectra; (B) Pt0 4f7/2 binding energies; (C) schematic illustration of the electron transfer from Cu to Pt atoms.


14

Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To date, it has been generally accepted that the modification of Pt surface electronic structures coming from geometric structures such as lattice strain plays a significant role in ORR activity enhancement in binary alloys catalysts.33 Therefore, XPS characterizations were conducted to explore the Pt electronic information, as represented in Figure 5A. It is obvious that Pt consists of multiple chemical valences such as Pt0 and Pt2. Clearly, Pt0 dominates among the above components by comparing their relative intensities, since Pt0 can provide more active sites for ORR than Pt2.

34

Thus, the fitted Pt0 binding energies represented by Pt0 4f7/2 are displayed in

Figure 5B, from which we can see that Pt 4f7/2 peaks of the Pt-Cu/C catalysts shift to lower binding energy compared to that of Pt/C catalyst. This is because Pt atoms will withdraw the electrons from Cu in the alloys due to the higher electronegativity of Pt (2.28) than that of Cu (1.90).35 Considering that the binding energy is related to Pt 5d electron density,36-37 we conclude that 5d electrons of Pt increase and the unoccupied 5d orbital decrease, as illustrated by Figure 5C. Moreover, the Pt 4f7/2 peak positions of these alloy catalysts move along the lower binding energy direction with the increase of lattice contraction percentage, suggesting that Pt 5d electrons also increase accordingly.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

Figure 6. (A) Partial enlarged drawing of the Pt L3-edge XANES spectra in Figure 2B; (B) WL intensity of these catalysts. To further verify the 5d electron densities in the Pt-based catalysts, XAS measurements were performed on these catalysts. The partial enlarged drawing of the normalized XANES spectra for Pt L3-edge (Figure 2B) is depicted in Figure 6A. As is seen, the threshold energy (E0) and the maximum energy (Epeak) for the Pt-Cu alloys are similar to those of the corresponding Pt/C catalyst, demonstrating the metallic nature of Pt atoms on the alloy samples.27 Besides, the appearance of strong peaks for the white line (WL) reveals that the dipole allows a photoelectron transfer from 2p core to unoccupied 5d state.38 It is reported that the WL intensity is in proportion to unoccupied state density in the Pt 5d orbitals.39-40 That is, the lower WL intensity, the less Pt 5d orbital vacancies. As observed in Figure 6B, the WL intensity appears to decrease with the increase of lattice contraction percentage, indicating that Pt 5d orbital vacancies reduce accordingly. Namely, the Pt 5d electrons increase with the increase of lattice contraction percentage, which is highly consistent with the XPS results. It is believed that the modification of electronic structures caused by the Pt-Pt bond contraction has a vital influence on the adsorption and desorption of oxygen-containing species, and then affecting the ORR kinetics.41


16

Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (A) The proposed ORR mechanism on Pt-Cu catalysts in alkaline environments; (B) schematic illustration of the overall ORR process catalyzed by the Pt-Cu alloys. According to the previous studies about the complete ORR process that involving multi-step electron transfer,

42-44

a possible ORR kinetic mechanism via four-electron transfer pathway in

alkaline electrolyte is schematically depicted in Figure 7A. It is believed that the active oxygen adsorption (step 1) or the H2O desorption (step 6) on the catalyst surface is crucial ratedetermining step of the ORR.33, 45 Therefore, the ORR activity in nature depends on the binding force of the catalysts to the oxygen-containing species. The overall ORR process catalyzed by the Pt-Cu alloys is illustrated in Figure 7B. According to the Sabatier principle, 46-47 which refers to that the reaction rate reaches maximized when the catalyst binds atoms and molecules with a moderate strength. In terms of the ORR process, the activity will be the optimum when the binding force between the catalyst and the oxygen-containing species is moderate. If the binding is too strong, more oxygenated intermediates (O*、OH*、OOH*, * refers to a site on the surface) will accumulate on the catalyst surface. As a result, the H2O desorption is so difficult that it becomes the rate-determining step of ORR, which in turn blocks the active sites for further


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

oxygen adsorption, decreasing the ORR rate.45 Conversely, if the binding is too weak, the active oxygen adsorption is so hard that it turns into the rate-determining step of ORR. On this occasion, the ORR rate will also be restricted on account of the increased activation barrier for oxygen dissociation.20, 48 As proved by the previous XPS and XAS results, Pt 5d electrons increase with the increase of the lattice contraction percentage. It is believed that the increase of d electrons lowers the d-band center, which reduces the state density of Fermi level and diminishes the chemical adsorption strength.41, 49 Thus, in our case, the oxygen-containing species adsorption on catalyst surface becomes weaker along with the increase of the lattice contraction percentage. In the beginning, as the lattice contraction percentage increase, the Pt-Pt bond length decrease, the active oxygen is inclined to be adsorbed on catalyst surface in a “bridge side-on two sides” style, which is in favor of the O-O bond breaking to accelerate the ORR rate.50 Besides, the binding between the catalyst and oxygenated intermediates becomes weaker owing to the weaker adsorption ability and the enhanced repulsive force of oxygenated intermediates. Ultimately, the ORR activity increases because of more available active sites. However, when beyond the critical lattice contraction percentage, the active oxygen adsorption becomes weaker, inhibiting the breakage of O-O bond, and finally hampering the ORR process. Therefore, the relationship between lattice contraction percentage and the ORR activity displays a volcano type. 4.

CONCLUSION In this work, we fabricated a series of Pt-Cu alloy NPs with similar particle sizes but different

compositions by the conventional polyol method. By using them as examples, we studied the dependent relationship between quantitative lattice contraction and electro-catalytic activity. The results manifest that the ORR activities of Pt-Cu alloy catalysts exhibit a more well-defined relationship toward the lattice contraction percentage than the alloy composition. XPS and XAS


18

Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


analysis reveal that Pt 5d electrons increase with the increase of lattice contraction percentage, bringing about the decline of d-band center, further weakening the adsorption strength of oxygen-containing species. At first, as the lattice contraction percentage increase, the Pt-Pt bond distance declines, the active oxygen tends to be adsorbed on catalyst surface in a “bridge side-on two sides” style. It is in favor of the O-O bond breaking to accelerate the ORR rate. In addition, the adsorption of oxygenated intermediates on the catalyst surface becomes weaker due to the weaker adsorption ability and the enhanced repulsive force of oxygenated intermediates. Thus, the ORR activity improves because the weaker binding of oxygenated intermediates releases more active sites. However, when beyond the critical lattice contraction percentage, the active oxygen adsorption becomes weaker and the ORR kinetics is hampered. Hence, the correlation between lattice contraction percentage and ORR activity displays a volcano type. This study comes up with an effective descriptor to bridge the ORR activity and atomic composition, which is conductive to understand profoundly the composition-structure-activity relationship of Ptbased alloys. ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. The materials contain additional SEM image, CV curves, etc. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Liu), Tel: +86-10-64411301.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

[email protected] (F. Wang), Tel: +86-10-64451996 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank all the staff of 1W1B-XAFS beamline of Beijing Synchrotron Radiation Facility (BSRF) for the XAS measurements. This work was financially supported by the National Natural Science Funds of China (Grant No. 51572013). REFERENCES (1)

Liu, Y.; Lehnert, W.; Janßen, H.; Samsun, R. C.; Stolten, D. A Review of HighTemperature Polymer Electrolyte Membrane Fuel-Cell (HT-PEMFC)-Based Auxiliary Power Units for Diesel-Powered Road Vehicles. J. Power Sources 2016, 311, 91-102.

(2)

Geboes, B.; Mintsouli, I.; Wouters, B.; Georgieva, J.; Kakaroglou, A.; Sotiropoulos, S.; Valova, E.; Armyanov, S.; Hubin, A.; Breugelmans, T. Surface and Electrochemical Characterisation of a Pt-Cu/C Nano-Structured Electrocatalyst, Prepared by Galvanic Displacement. Appl. Catal. B: Environ. 2014, 150, 249-256.

(3)

Chattot, R.; Asset, T.; Bordet, P.; Drnec, J.; Dubau, L.; Maillard, F. Beyond Strain and Ligand Effects: Microstrain-Induced Enhancement of the Oxygen Reduction Reaction Kinetics on Various PtNi/C Nanostructures. ACS Catal. 2016, 7, 398-408.

(4)

Miyatake, K.; Shimizu, Y. Pt/Co Alloy Nanoparticles Prepared by Nanocapsule Method Exhibit a High Oxygen Reduction Reaction Activity in the Alkaline Media. ACS Omega 2017, 2, 2085-2089.


20

Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(5)


Wang, Y.-J.; Zhao, N.; Fang, B.; Li, H.; Bi, X. T.; Wang, H. Carbon-Supported Pt-Based Alloy Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells: Particle size, Shape, and Composition Manipulation and Their Impact to Activity. Chem. Rev. 2015, 115, 3433-3467.

(6)

Choi, S.-I.; Lee, S.-U.; Kim, W. Y.; Choi, R.; Hong, K.; Nam, K. M.; Han, S. W.; Park, J. T. Composition-Controlled PtCo Alloy Nanocubes with Tuned Electrocatalytic Activity for Oxygen Reduction. ACS Appl. Mater. Interfaces 2012, 4, 6228-6234.

(7)

Mani, P.; Srivastava, R.; Strasser, P. Dealloyed Binary PtM3 (M= Cu, Co, Ni) and Ternary PtNi3M (M= Cu, Co, Fe, Cr) Electrocatalysts for the Oxygen Reduction Reaction: Performance in Polymer Electrolyte Membrane Fuel Cells. J. Power Sources 2011, 196, 666-673.

(8)

Shao, M.; Chang, Q.; Dodelet, J. P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594-3657.

(9)

Zhao, Y.; Liu, J.; Zhao, Y.; Wang, F.; Song, Y. Pt-Co Secondary Solid Solution Nanocrystals Supported on Carbon as Next-Generation Catalysts for the Oxygen Reduction Reaction. J. Mater. Chem. A 2015, 3, 20086-20091.

(10) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. Role of Strain and Ligand Effects in the Modification of the Electronic and Chemical Properties of Bimetallic Surfaces. Phys. Rev. Lett. 2004, 93, 156801.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

(11) Deng, Y.-J.; Tripkovic, V.; Rossmeisl, J.; Arenz, M. Oxygen Reduction Reaction on Pt Overlayers Deposited onto a Gold Film: Ligand, Strain, and Ensemble Effect. ACS Catal. 2015, 6, 671-676. (12) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6, 241-247. (13) Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J. Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction: An in Situ XANES and EXAFS Investigation. J. Electrochem. Soc. 1995, 142, 1409-1422. (14) Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Nørskov, J. K. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chem. Int. Ed. 2006, 118, 2963-2967. (15) Nørskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Towards the Computational Design of Solid Catalysts. Nat. Chem. 2009, 1, 37-46. (16) Jia, Q.; Liang, W.; Bates, M. K.; Mani, P.; Lee, W.; Mukerjee, S. Activity Descriptor Identification for Oxygen Reduction on Platinum-Based Bimetallic Nanoparticles: In Situ Observation of the Linear Composition-Strain-Activity Relationship. ACS Nano 2015, 9, 387-400. (17) Zhang, J.; Lima, F.; Shao, M.; Sasaki, K.; Wang, J.; Hanson, J.; Adzic, R. Platinum Monolayer on Nonnoble Metal-Noble Metal Core-Shell Nanoparticle Electrocatalysts for O2 Reduction. J. Phys. Chem. B 2005, 109, 22701-22704.


22

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Mukerjee, S.; Srinivasan, S. Enhanced Electrocatalysis of Oxygen Reduction on Platinum Alloys in Proton Exchange Membrane Fuel Cells. J. Electroanal. Chem. 1993, 357, 201-224. (19) Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J. Effect of Preparation Conditions of Pt Alloys on Their Electronic, Structural, and Electrocatalytic Activities for Oxygen Reduction-XRD, XAS, and Electrochemical Studies. J. Phys. Chem. 1995, 99, 4577-4589. (20) Strasser, P.; Koh, S.; Anniyev, T.; Greeley, J.; More, K.; Yu, C.; Liu, Z.; Kaya, S.; Nordlund, D.; Ogasawara, H. Lattice-Strain Control of the Activity in Dealloyed Core-Shell Fuel Cell Catalysts. Nat. Chem. 2010, 2, 454-460. (21) Yang, J.; Chen, X.; Yang, X.; Ying, J. Y. Stabilization and Compressive Strain Effect of AuCu Core on Pt Shell for Oxygen Reduction Reaction. Energy Environ. Sci. 2012, 5, 8976-8981. (22) Zhang, X.; Lu, G. Computational Design of Core/Shell Nanoparticles for Oxygen Reduction Reactions. J. Phys. Chem. Lett. 2013, 5, 292-297. (23) Dou, S.; Tao, L.; Huo, J.; Wang, S.; Dai, L. Etched and Doped Co9S8/Graphene Hybrid for Oxygen Electrocatalysis. Energy Environ. Sci. 2016, 9, 1320-1326. (24) Lin, R.; Zhao, T.; Shang, M.; Wang, J.; Tang, W.; Guterman, V. E.; Ma, J. Effect of Heat Treatment on the Activity and Stability of PtCo/C Catalyst and Application of In-Situ X-ray Absorption Near Edge Structure for Proton Exchange Membrane Fuel Cell. J. Power Sources 2015, 293, 274-282.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

(25) Zhao, Y.; Liu, J.; Zhao, Y.; Wang, F. Composition-Controlled Synthesis of CarbonSupported Pt-Co Alloy Nanoparticles and the Origin of their ORR Activity Enhancement. Phys. Chem. Chem. Phys. 2014, 16, 19298-19306. (26) Liu, J.; Xu, C.; Liu, C.; Wang, F.; Liu, H.; Ji, J.; Li, Z. Impact of Cu-Pt Nanotubes with a High Degree of Alloying on Electro-Catalytic Activity Toward Oxygen Reduction Reaction. Electrochim. Acta 2015, 152, 425-432. (27) Cheng, N.; Stambula, S.; Wang, D.; Banis, M. N.; Liu, J.; Riese, A.; Xiao, B.; Li, R.; Sham, T.-K.; Liu, L.-M. Platinum Single-Atom and Cluster Catalysis of the Hydrogen Evolution Reaction. Nat. Commun. 2016, 7, 13638. (28) Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D. Structurally Ordered Intermetallic Platinum-Cobalt Core-Shell Nanoparticles with Enhanced Activity and Stability as Oxygen Reduction Electrocatalysts. Nat. Mater. 2013, 12, 81-87. (29) Chen, G.; Xu, C.; Huang, X.; Ye, J.; Gu, L.; Li, G.; Tang, Z.; Wu, B.; Yang, H.; Zhao, Z. Interfacial Electronic Effects Control the Reaction Selectivity of Platinum Catalysts. Nat. Mater. 2016, 15, 564-569. (30) Asano, M.; Kawamura, R.; Ren, S.; Todoroki, N.; Wadayama, T. Oxygen Reduction Reaction Activity for Strain-Controlled Pt-Based Model Alloy Catalysts: Surface Strains and Direct Electronic Effects Induced by Alloying Elements. ACS Catal. 2016, 6, 52855289.


24

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Yang, H.; Walter Vogel; Claude Lamy, A.; Alonsovante, N. Structure and Electrocatalytic Activity of Carbon-Supported Pt-Ni Alloy Nanoparticles Toward the Oxygen Reduction Reaction. J. Phys. Chem. B 2004, 108, 11024-11034. (32) Gan, L.; Yu, R.; Luo, J.; Cheng, Z.; Zhu, J. Lattice Strain Distributions in Individual Dealloyed Pt-Fe Catalyst Nanoparticles. J. Phys. Chem. Lett. 2012, 3, 934-938. (33) Hyman, M. P.; Medlin, J. W. Effects of Electronic Structure Modifications on the Adsorption of Oxygen Reduction Reaction Intermediates on Model Pt (111)-Alloy Surfaces. J. Phys. Chem. C 2007, 111, 17052-17060. (34) Wu, P.; Zhang, H.; Qian, Y.; Hu, Y.; Zhang, H.; Cai, C. Composition-and Aspect-RatioDependent

Electrocatalytic

Performances

of

One-Dimensional

Aligned

Pt-Ni

Nanostructures. J. Phys. Chem. C 2013, 117, 19091-19100. (35) Su, L.; Shrestha, S.; Zhang, Z.; Mustain, W.; Lei, Y. Platinum-Copper Nanotube Electrocatalyst with Enhanced Activity and Durability for Oxygen Reduction Reactions. J. Mater. Chem. A 2013, 1, 12293-12301. (36) Qin, Y. H.; Li, Y.; Lv, R. L.; Wang, T. L.; Wang, W. G.; Wang, C. W. Enhanced Methanol Oxidation Activity and Stability of Pt Particles Anchored on Carbon-Doped TiO2 Nanocoating Support. J. Power Sources 2015, 278, 639-644. (37) Toda, T.; Igarashi, H.; Watanabe, M. Enhancement of the Electrocatalytic O2 Reduction on Pt-Fe Alloys. J. Electroanal. Chem. 1999, 460, 258-262.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

(38) Cheng, N.; Banis, M. N.; Liu, J.; Riese, A.; Mu, S.; Li, R.; Sham, T.-K.; Sun, X. Atomic Scale Enhancement of Metal-Support Interactions Between Pt and ZrC for Highly Stable Electrocatalysts. Energy Environ. Sci. 2015, 8, 1450-1455. (39) Miller, J. T.; Schreier, M.; Kropf, A. J.; Regalbuto, J. R. A Fundamental Study of Platinum Tetraammine Impregnation of Silica: 2. The Effect of Method of Preparation, Loading, and Calcination Temperature on (Reduced) Particle Size. J. Catal. 2004, 225, 203-212. (40) Xin, L.; Yang, F.; Rasouli, S.; Qiu, Y.; Li, Z.-F.; Uzunoglu, A.; Sun, C.-J.; Liu, Y.; Ferreira, P.; Li, W. Understanding Pt Nanoparticle Anchoring on Graphene Supports through Surface Functionalization. ACS Catal. 2016, 6, 2642-2653. (41) Hammer, B.; Nørskov, J. K. Theoretical Surface Science and Catalysis-Calculations and Concepts. Adv. Catal. 2000, 45, 71-129. (42) Koper, M. T. Thermodynamic Theory of Multi-Electron Transfer Reactions: Implications for Electrocatalysis. J. Electroanal. Chem. 2011, 660, 254-260. (43) Gu, W.; Liu, J.; Hu, M.; Wang, F.; Song, Y. La2O2CO3 Encapsulated La2O3 Nanoparticles Supported on Carbon as Superior Electrocatalysts for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2015, 7, 26914-26922. (44) Yeager, E. Electrocatalysts for O2 Reduction. Electrochim. Acta 1984, 29, 1527-1537. (45) Xia, W.; Mahmood, A.; Liang, Z.; Zou, R.; Guo, S. Earth-Abundant Nanomaterials for Oxygen Reduction. Angew. Chem. Int. Ed. 2016, 55, 2650-2676.


26

Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(46) Medford, A. J.; Vojvodic, A.; Hummelshøj, J. S.; Voss, J.; Abild-Pedersen, F.; Studt, F.; Bligaard, T.; Nilsson, A.; Nørskov, J. K. From the Sabatier Principle to a Predictive Theory of Transition-Metal Heterogeneous Catalysis. J. Catal. 2015, 328, 36-42. (47) Shao, M.; Odell, J. H.; Peles, A.; Su, D. The Role of Transition Metals in the Catalytic Activity of Pt Alloys: Quantification of Strain and Ligand Effects. Chem. Commun. 2014, 50, 2173-2176. (48) Hammer, B.; Norskov, J. Why Gold is the Noblest of all the Metals. Nature 1995, 376, 238. (49) Hammer, B.; Morikawa, Y.; Nørskov, J. K. CO Chemisorption at Metal Surfaces and Overlayers. Phys. Rev. Lett. 1996, 76, 2141. (50) Cheng, F.; Chen, J. Metal-Air Batteries: From Oxygen Reduction Electrochemistry to Cathode Catalysts. Chem. Soc. Rev. 2012, 41, 2172-2192.

Table of Contents Graphic


27

Dependent Relationship between Quantitative Lattice Contraction and

Recommend Documents